Labour leader David Cunliffe has launched an attack on Conservative Party leader Colin Craig amid growing speculation of an electoral deal with National.

Prime Minister John Key told media this week he would be looking to make three deals to give minor-party candidates a chance at winning seats, and would announce them before the September 20 election.

It is widely understood one of those deals would be with Craig, who is set to announce this month the electorate he will be standing in.

Craig has already indicated his preference to stand in one of the northern Auckland electorates - East Coast Bays, which is held by Foreign Affairs Minister Murray McCully; Upper Harbour, in which National is standing Social Development Minister Paula Bennett; and Rodney, which is held by National back-bench MP Mark Mitchell.

If National gave way for Craig to win a seat and the Conservative's gained more than 1.2 per cent of the vote, an extra MP could be brought into Parliament to form a coalition with National.

Cunliffe today said the situation with Craig was a "strange turn of events".

"I wonder whether the people of East Coast Bays like the fact that their choices are being taken away from them, that MMP is being manipulated and they're being told they have to vote for somebody who basically thinks the earth is flat," he said on Firstline.

"That is a very, very strange turn of events and I think it underwhelms public confidence in the MMP system.

"I also think the prime minister should be aware that something like 75 per cent of New Zealanders object to coat-tailing.

"Labour is opposed to coat-tailing, we've got a bill in the House to end it, we've invited the Government to support the bill and so far they've declined. Now we see why - because they want to use coat-tailing to get fringe parties up into Parliament by gifting them a seat, most likely East Coast Bays."

Meanwhile, the Conservative Party coffers were steadily filling thanks to Craig, who has donated $100,000 - his third donation in just over three weeks.

Electoral Commission records show Colin Craig also injected $55,000 into his Conservative Party just two weeks ago. That donation was split into two - $25,000 received on May 13, and $30,000 received on May 28. All-up, he's made eight donations since October last year, ranging from $10,000 to $550,000.

Craig said donations would come from other sources.

"It's my expectation that we'll see some pretty significant donations, but obviously still to happen, though it's on track," he said, but he wasn't expecting many.

"Most political donations really, you've got a low level of donations from supporters and members and we expect to get that. And we did pretty well on that last time, but we expect to get more given the numbers, but the lion's share of it tends to come from a relatively small number of donors."

Big money was set to feature more prominently among minor parties this election.

A $3.25m donation from Kim Dotcom's to the Internet-Mana alliance was registered with the Electoral Commission last weekend.